Progress made in such piezoelectric materials having large and stable piezoelectricity as represented by a PZT (lead zirconate titanate) based piezoelectric ceramic, a piezoelectric transducer using the same, and a semiconductor transmit-receive circuit highly adaptable to the piezoelectric transducer has contributed to remarkable development and widespread use of an ultrasonic technology during the latter half of the 20th century. In the early years of the 20th century, the human race started an attempt to transmit and receive ultrasonic waves by utilizing a piezoelectric effect that was discovered by the Curie brothers in the latter half of the 19th century. However, even though a rock crystal of which they discovered the piezoelectric effect has piezoelectric properties so stable as to enable it to be used in a clock even today, the rock crystal is low in electro-mechanical conversion efficiency, and in particular, sensitivity of a signal-receiving transducer using the same is low, which has turned out to be its main drawback. There has since been found a Rochelle salt that is very high in electro-mechanical conversion efficiency. The Rochelle salt, however, has since been found prone to undergo deliquescence, posing a problem with crystal stability, so that particular caution has been required in order to enable it to obtain a stable piezoelectric property. Nevertheless, because a substitute for the Rochelle salt was unavailable during World War II, an ultrasonic transducer was completed by use of the Rochelle salt, and subsequently, a sonar was developed by use of the ultrasonic transducer. Immediately after World War II, barium titanate whose electro-mechanical conversion efficiency is high and stable was found having piezoelectricity. Since barium titanate is a ceramic, it has an advantage of high flexibility in product shape, and a concept called “piezoelectric ceramics” was thereby born. Subsequently, lead zirconate titanate (PZT) ceramic higher in Curie point than barium titanate, thereby having more stable piezoelectric properties, was discovered late in the 20th century, and has since come into widespread use for the ultrasonic transducer in commercial application up to now.
Meanwhile, there is the need for an electronic circuit accompanying the ultrasonic transducer, for driving the ultrasonic transducer at the time of signal transmission, and amplifying electric signals received by the ultrasonic transducer at the time of signal reception, and a circuit made up of vacuum tubes was in use during a time period from the days of the sonar developed during World War II, and up to 1970s. In comparison with an electronic circuit for audio-frequency range, in which semiconductor was adopted early on after a transistor was invented immediately after World War II, an electronic circuit for ultrasonic waves had a higher operational frequency range, so that adoption of semiconductor for the electronic circuit for the ultrasonic waves was delayed by about 20 years. With a drive circuit for signal transmission, in particular, an operation at a high voltage is required, so that adoption of semiconductor for the drive circuit had to wait until commercial application of a high-speed thyristor, and further, widespread use of the high-speed thyristor had to wait until commercial application of a high-voltage-resistant field effect transistor (FET).
As described above, a piezoelectric ceramic-based ultrasonic transducer presently represents the majority of ultrasonic transducers that are in commercial application. With the aim of replacing the piezoelectric ceramic-based ultrasonic transducer, R and D on the construction of a microscopic diaphragm-based transducer by use of a technology for micro-machining semiconductor, as represented by one described in Proceedings of 1994 IEEE Ultrasonics Symposium, pp. 1241-1244, were started from 1990s onwards.
According to a typical basic structure thereof, a capacitor is formed by electrodes 2, 3 that are provided on a substrate 1, and a diaphragm 5, respectively, with a void 4 interposed therebetween. When a voltage is applied across those electrodes, electric charges with polarities opposite to each other are induced on the respective electrodes, thereby exerting an attracting force on each other, so that the diaphragm undergoes displacement. If the outer side of the diaphragm is in contact with water and a living body at this point in time, acoustic waves are emitted into those media, which is the principle underlying electro-mechanical conversion in signal transmission. On the other hand, if a given electric charge is kept induced on the respective electrodes by applying a DC bias voltage thereto, and vibration is forcefully given from a medium in contact with the diaphragm, thereby causing the diaphragm to undergo displacement, a voltage corresponding to the displacement is additionally generated. The principle underlying the electro-mechanical conversion in signal reception, described in the latter case, is the same as that for a DC bias capacitor microphone for use as a microphone in an audible sound range. The diaphragm-based transducer is made up of a mechanically hard material such as silicon, but features excellent acoustic impedance matching with a mechanically soft material such as the living body, water, and so forth because the diaphragm-based ultrasonic transducer has a diaphragm structure with the void provided on the back surface of the diaphragm. In the case of a conventional piezoelectric transducer using PZT, acoustic impedance is constant as an intrinsic physical property value of material, and in contrast thereto, apparent acoustic impedance of the diaphragm structure reflects not only material thereof but also a structure thereof. Accordingly, there is obtained flexibility in designing so as to match a target. Further, combination of the transducer with the transmit/receive circuit as described in the foregoing is a point of importance for the transducer, and construction of the transducer by use of silicon for the substrate thereof will lead to a feature in that a signal reception circuit and a signal transmission circuit can be provided in close proximity to the transducer so as to be integral therewith, respectively. Progress in development of the transducer has since been made, having lately reached a level comparable in respect of sensitivity of signal transmission/reception to that of the conventional piezoelectric transducer using PZT.
In J. Acoust. Soc. Am. vol. 75, 1984, pp. 1297-1298, there is disclosed an electret transducer using a semiconductor diaphragm structure. With the electret transducer, an insulating layer 5 with electric charges stored therein is provided at least either between an electrode 3 on a side of the transducer, adjacent to the diaphragm in FIG. 1, and the void 4, or between an electrode 2 on a side of the transducer, adjacent to the substrate, and the void 4. For a constituent material making up the insulating layer with the electric charges stored therein, use is made of a silicon compound film such as a silicon oxide film, silicon nitride film, and so forth, or a stack thereof, as shown in J. Acoust. Soc. Am. vol. 75, 1984, pp. 1297-1298, and IEEE Transactions on Dielectrics and Electrical Insulation vol. 3, No. 4, 1996, pp. 494-498. The insulating layer composed of those silicon compounds is formed by means of vapor growth by use of a process represented by CVD (Chemical Vapor Deposition), and it is possible to trap the electric charges not only on the surface of the compound layer but also in the compound layer by controlling magnitude of crystalline defects. For this purpose, by causing the insulating layer to undergo electrification under a high electric field beforehand, the electret transducer is used as an electro-acoustic transducer device having no necessity for the DC bias voltage.